Sulfur containing compounds present in oil derivatives and fuels can lead to air pollution and acid rains. The presence of high concentrations of sulfur containing compounds in oil derivatives can also lead to corrosion in containers, reactors, pipes and, metal joints and damaging the catalysts in downstream processes.
Gasoline and gas oil account for about 75 to 80% of the total refinery products. Most of the desulphurization processes are therefore meant to remove sulfur containing compounds from the refinery products. Furthermore, low-sulfur gas oils are harder to produce due because gas oils produced through catalytic fluidized bed cracking and coke formation contains about 2.5% wt. of sulfur with low cetane number, high density and high aromatic content.
Removal of sulfur is possible to some extent by conventional processes utilizing CoMo, NiMo, and CoNiMo catalysts over alumina support. Alumina is the most widely used support of hydrodesulphurization catalysts. Notable feature of alumina supports is their ability to provide high dispersion of the active metal components.
One of the drawbacks of using alumina as a support material during hydrodesulphurization process is formation of coke, which reduces the catalyst's lifetime and leads to frequent and compulsory change of the catalytic bed in the reactor. Another drawback of using alumina as a support material during hydrodesulphurization process is the strong interaction of the alumina support with metal oxides, which decreases the sulfidation yield. This can lead to decrease in the catalyst's surface area.
EP 0 969 075 A1 describes the use of group VI and group VIII metals over alumina support for preparation of hydrodesulfurization catalysts for diesel fractions. The most active catalyst (B1) is supported on alumina with a surface area of 372 m2/g and a pore volume of 0.65 cm3/g. The catalyst contained 25% wt. of molybdenum and cobalt oxides, 3% wt. of P2O5, and 5% wt. of HY zeolite having a surface area of 267 m2/g, pore volume of 0.45 cm3/g, and an average pore size of 7.8 nm. The hydrodesulfurization of gas oil was carried out at 350° C., 3.5 MPa with a liquid hourly space velocity (LHSV) of 1.5 h−1. This process reduced the sulfur content of the feed from 13500 ppm to 140 ppm.
EP 0 969 075 A1 also describes catalyst E based on alumina support with a surface area of 243 m2/g and a pore volume of 0.41 cm2/g. The catalyst contained 30% wt. of molybdenum and cobalt oxides, 3% wt. of P2O5, and 5% wt. of HY zeolite having a surface area of 267 m2/g, a pore volume of 0.455 cm2/g, and an average pore size of 7.8 nm. The hydrodesulfurization of gas oil process was carried out at a temperature of 340° C., a pressure of 3.5 MPa, and an LHSV of 1.5 h−1. This process reduced the sulfur content of the feed from 13500 ppm to 228 ppm.
According to example 15 of WO 99/673455 a catalyst is supported on alumina with a surface area of 214 m2/g, pore volume of 0.41 cm2/g, and pore diameter of 7.6 mm. The catalyst contained 20.44% wt. of molybdenum and cobalt oxides. According to WO 99/673455, this catalyst reduced the sulfur content of the feed from 12286 ppm to 115 ppm when the hydrodesulfurization of gas oil process was carried out under at a pressure of 4 MPa, at a temperature of 354° C., and an LHSV of 1.35 h−1.
EP 1 832 645 A1 discloses a catalyst containing 16% wt. of molybdenum and cobalt oxides. According to EP 1 832 645 A1, this catalyst reduced the sulfur content of the feed from 11600 ppm to 6.5 ppm when the hydrodesulfurization of gas oil process was carried out at a temperature of 345° C., pressure of 7 MPa, H2/feed of 440 Ncm3/cm3, and an LHSV of 1.2 h−1.
US Pat. Appl. Pub. No. 2002/0070147 discloses a catalyst containing 25% wt. of molybdenum and cobalt oxides and 3% wt of P2O5. According to US Pat. Appl. Pub. No. 2002/0070147, this catalyst reduced the sulfur content of the feed from 12000 ppm to 21 ppm when the hydrodesulfurization of gas oil process was carried out at a temperature of 330° C., a pressure of 4 MPa, H2/feed of 300 Ncm3/cm3, and a space velocity of 2 h−1 after being sulfidized with a 2.5% DMDS solution.
The general, the common drawbacks of hydrodesulfurization catalysts based on alumina are clogging of the catalyst and non-homogeneous distribution of active metals. Given that high metal loadings are not possible in these catalysts, the resulting catalysts cannot be used in high concentrations of dibenzothiophenes and alkyl dibenzothiophenes. Accordingly, hydrodesulfurization of gas oil process requires low LHSV, high pressure, and high H2/feed ratios, which in turn increase coke formation. Coke formation reduces catalyst lifetime due to blockade of active sites of the catalyst.
Accordingly, a need exists for a hydrodesulfurization nanocatalyst that overcomes the problems of the prior art catalysts by having an improved catalyst activities and an increased hydrodesulfurization yield.